Method and device for operating mercury-process electrolytic cells

ABSTRACT

THE REFINED BRINE RECIRCULATED TO A MERCURY-CATHODE CELL IS INTRODUCED INTO THE CELL IN THE FORM OF A FALLING LAMINARFLOW CURTAIN ALONG ALMOST THE ENTIRE INNER SURFACE OF AN INLET TRANSVERSE WALL OF THE CELL ABOVE THE RECOVERED MERCURY INLET, THE LAMINAR-FLOW CURTAIN OF BRINE THEREBY REACHING AND THROUGHLY WASHING THE ENTERING MERCURY TO PREVENT ANY ALKALINE WASH WATER FLOWING ABOVE THE MERCURY FROM REACHING THE GRAPHITE ANODE. THIS LAMINAR-FLOW CURTAIN IS FORMED BY A LONG AND NARROW BRINE INLET PROVIDED IN AND EXTENDING HORIZONTALLY ACROSS THE FULL WIDTH OF THE TRANSVERSE WALL AT A POSITION SUFFICIENTLY HIGH ABOVE THE SURFACE LEVEL OF THE ELECTROLYTE IN THE CELL.

Feb. 2, 1971 HIROS HI SHIBATA 3,560,355

1 METHOD AND DEVICE FOR OPERATING MERCURY-PROCESS ELECTROLYTIC CELLS Filed April 5, 1968 2 Sheets-Sheet 2 Huwsm S-HIBATA. TERUO mm Mb SHIGEJ'I KU MAKI INVENTORS Bark/@1041. W

United States Patent O 1 3,560,355 METHOD AND DEVICE FOR OPERATING MERCURY-PROCESS ELECTROLYTIC CELLS Hiroshi Shibata, Teruo Imai, and Shigeji Kumaki, Iwakishi, Japan, assignors to Kureha Kagaku Kogyo Kabushiki Kaisha, Tokyo-to, Japan Filed Apr. 3, 1968, Ser. No. 718,438 Claims priority, application Japan, Apr. 19, 1967, 42/24,560; Nov. 24, 1967, 42/75,371 Int. Cl. C01d N08 US. Cl. 204-99 4 Claims ABSTRACT OF THE DISCLOSURE The refined brine recirculated to a mercury-cathode cell is introduced into the cell in the form of a falling laminarflow curtain along almost the entire inner surface of an inlet transverse wall of the cell above the recovered mercury inlet, the laminar-flow curtain of brine thereby reaching and thoroughly washing the entering mercury to prevent any alkaline wash water flowing above the mercury from reaching the graphite anode. This laminar-flow curtain is formed by a long and narrow brine inlet provided in and extending horizontally across almost the full width of the transverse Wall at a position sufficiently high above the surface level of the electrolyte in the cell.

BACKGROUND OF THE INVENTION This invention relates generally to the field of mercury-process electrolysis of brine and more particularly to new improvements in mercury-process electrolytic cells and in the operation thereof.

In a mercury-process electrolysis operation in an apparatus of the type including a mercury-cathode cell, a denuding tower and a mercury circulation pump, the mercury which has left the denuding tower still contains some alkali in a residual state. Accordingly, water is added to this mercury either before it enters the mercury pump or after it has been delivered by the pump thereby to wash the mercury, which is then separated from the wash water and recirculated to the electrolytic cell.

However, complete separation of continuously flowing mercury and wash water is extremely diflicult, and, normally, some wash water containing an alkali such as caustic soda unavoidably accompanies the mercury and enters the electrolytic cell. When the alkaline wash water thus enters the electrolytic cell, it reacts with the chlorine generated within the cell to form oxidising salts such as sodium hypochlorite and sodium chlorate, which oxidise and erode the anode.

The alkaline wash water which has accompanied the mercury and entered the electrolytic cell undergoes almost no mixing with the electrolyte, and most of this wash water flows as a laminar flow below the electrolyte and is conducted, together with the mercury, to the lower part of the graphite anode. Consequently, the part of the anode in the vicinity of the mercury inlet of the electrolytic cell is particularly subjected to severe local erosion, whereby the anode is consumed in an uneven manner (as indicated in FIG. 1(a) described hereinafter).

As a result, it is necessary to stop the operation of the electrolytic cell frequently to replace the anode although no appreciable erosive consumption has occurred in the other parts of the anode. Consequently, a great loss is incurred not only because of the high rate of consumption of the anode but also because of the shutdowns necessary for replacements of the anode. In addition, each shutdown gives rise to erosion of the electrolytic cell leading to other losses such as the necessity for additional electrolyte, electric power, repair and replenishment materials (such as packings) and labour.

An obvious countermeasure for this problem is to neutralise the alkaline wash water which has entered the electrolytic cell before it reaches the anode. This countermeasure, however, is accompanied by further oifiiculties and limitations. More specifically, it is not possible to employ an operation wherein the mercury surface develops wave motion, or the body of mercury is broken and becomes discontinuous because of action such as vigorous agitation.

Furthermore, while most of the alkaline wash water can be neutralised by the conventionally practiced method of supplying acidic brine into a so-called top box of the electrolytic cell, such a top box as known heretofore is not desirable since it enlarges the size of the electrolytic cell by its additional bulk and, moreover, gives rise to an increase in the quantity of mercury needed for operation of the cell. Furthermore, this method in which a conventional top box is used is still inadequate in that it cannot completely prevent unneutralised alkaline wash water from reaching the anode as described in detail hereinafter.

SUMMARY OF THE INVENTION It is an object of the present invention to provide improvements in the method of supplying brine into the electrolytic cell and in the construction of the brine inlet thereby to prevent local erosion of the graphite anode in a mercury-process electrolytic cell (known also as a mercury-cathode cell).

According to the present invention, briefly summarised, there are provided, in a mercury-process electrolytic cell of the type referred to above, a brine supply method and device whereby the supply brine is supplied in the form of a falling laminar-flow curtain descending along substantially the entire inner surface of a transverse wall directly above a mercury inlet for introducing recovered mercury into the cell, the supply brine curtain thereby reaching the mercury thus introduced thereby to wash the entire upper surface of this mercury and the region thereabove.

The nature, principle, details, and utility of the present invention will be more clearly apparent from the following detailed description beginning with general considerations and concluding with a description of preferred embodiments of the invention, when read in conjunction with the accompanying drawings, in which like parts are designated by like reference numerals.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIGS. 1(a) and 1(b) are side elevational views showing states of erosion of anodes used in mercury-process electrolytic cells (the original states of the anodes prior to use being indicated by intermittent lines), FIG. 1(a) showing an anode used in a conventional electrolytic cell, and FIG. 1(b) showing an anode used in an electrolytic cell according to the invention;

FIG. 2 is a side elevational view, in vertical section, showing the essential structure of one example of a known electrolytic cell provided with a top box;

FIG. 3 is a side elevational view, in vertical section, showing the essential structure of one example of an electrolytic cell embodying the invention, the plane of the section being parallel to the mercury flow path;

FIG. 4 is an elevational view, in section taken along the plane indicated by line IV-IV in FIG. 3; and

FIG. 5 is a partial elevational view, in section similar to FIG. 4, showing another example of a preferred embodiment of the invention.

DETAILED DESCRIPTION In a known electrolytic cell provided with a top box as mentioned hereinabove and as illustrated in FIG. 2, brine 7 enters a top box 3 partitioned from the electrolytic chamber 1 by an inverted weir or baflie through a brine inlet 6 consisting of one or more pipes. On one hand, mercury 8 enters the bottom of the electrolytic cell below the top box 3 through a mercury inlet passageway 4 and under and past a mercury bafile 5. The brine 7 and the mercury 8 together pass under and past the brine bafile 2 and enter the electrolytic chamber 1.

At the same time, alkaline wash water 9 enters the mercury inlet passageway 4 together with the mercury 8 and some of this wash water 9 accompanies the mercury into the top box 3, most of this wash water flowing in a laminar flow between the mercury and the brine. Consequently, in the case where the flow path in the top box 3 is short, the neutralisation of the alkaline wash water is incomplete, and a portion of this wash water in an unneutralised state flows into the region below the anode 10 to cause the aforementioned anode erosion.

The present invention, as mentioned hereinbefore, contemplates overcoming this problem by supplying the brine to the electrolytic cell in a manner such that the brine is in a falling laminar-flow curtain along the entire surface of a transverse wall at the mercury inlet section of the electrolytic cell thereby to cause continuous washing and neutralising of the surface of the mercury entering the cell by the resulting vertical flow of the brine thus supplied.

The form of the downward flow of the supply brine herein specified by the term Falling laminar-flow curtain can be obtained, in general, by causing the brine newly supplied into the electrolyte brine remaining in the electrolytic cell to descend over the entire surface of the transverse wall in a laminar-flow separate from but within the remaining electrolyte brine. The existence of this separate laminar flow can be detected, for example, by adding a dye to the supply brine to colour the same and observing that the liquid thus coloured flows in a descending curtain in a thin film along the transverse Wall, which flow reaches the mercury surface to accomplish continuous washing of this mercury surface.

An electrolytic cell most suitable for obtaining such a descending laminar flow along a transverse wall is a cell, for example with a structure such that the supply brine flows into the electrolytic cell through a long and narrow gap which is at a level above the surface of the stagnant electrolyte at the transverse wall of the metcury inlet and is provided across the entire horizontal length of the transverse wall.

In one example of an electrolytic cell of this character according to the invention, as illustrated in FIGS. 3 and 4, mercury 12 and wash water 13 which have arrived through a mercury passageway 11 are separated at an inverted weir or baflle provided at the lower part of a transverse wall 14 of the electrolytic cell, where only the mercury is separated out and, flowing under the baflle 15 and into the electrolytic chamber 16, flows downstream (to the right as viewed in FIG. 3) along and above the bottom 17 of the electrolytic chamber.

On one hand, acidic supply brine 20 is supplied from a passageway 18 in the form of a trough disposed on the outer side of the upper part of the transverse wall 14, through a gap 19, provided in the upper part, preferably at the top as shown in the drawings, of the transverse wall, and into the electrolytic chamber 16. Since the surface 21 of the electrolyte within the electrolytic chamber is lower than the overflow lip of the gap 19, the supply brine first falls along the entire inner surface of the transverse wall 14. The kinetic energy of this falling brine is such that, although this brine curtain enters the electrolyte within the electrolytic cell, most of this brine flows along the surface of the transverse wall without mixing with the electrolyte and directly reaches the mercury surface.

Thus, even if some of the wash water 13 happens to pass under and past the baflle 15 to enter the electrolytic chamber interior, it becomes thoroughly mixed with and neutralised by the acidic supply brine flowing downward along the transverse wall and vertically colliding in film form against the mercury surface at the entrance of the wash water. Thus, the anode is shielded from the alkaline effect of the wash water, and local erosion of the graphite anode is prevented.

In the above described apparatus and operation, it is not necessary that the supply brine be only fresh brine supplied newly to the electrolytic cell, it being possible to supply brine in the same manner also in the case where a portion of dilute brine which has been used once in the electrolytic cell is mixed With fresh supply brine, and the resulting brine mixture is supplied to the electrolytic cell.

Furthermore, while it is necessary that brine inlet gap 19 have a transverse (horizontal) length such that curtain of brine reaching the mercury surface has a transverse length at least approximately equal to the transverse length of the mercury inlet aperture of the electrolytic cell, it is not necessary that the brine flow at the brine inlet be a continuous flow, the only requirement being that the descending brine assume a continuous film or curtain state before it reaches the mercury surface. Accordingly, the brine inlet gap 19 may be in the form of a discontinuous gap consisting of a plurality of intermittent divisional gaps as in the example illustrated in FIG. 5.

The utility of the present invention in practice will be apparent from the results of a comparative test as described below.

In an electrolytic cell without a top box similar to that illustrated in FIG. 3, the brine inlet was provided in a side surface of the electrolytic cell along the mercury flow path at a position near the mercury inlet. In this case, the anode in the vicinity of the mercury inlet was consumed unevenly as indicated in FIG. 1(a) when operated at a current of a/dm. for approximately three months, whereby the initial thickness of 150 mm. was reduced to a thickness of from 70 to 90 mm. at the upstream part of the anode.

In contrast, when an electrolytic cell provided with a brine inlet extending horizontally across the full width of the transverse wall above the mercury inlet as in the example illustrated in FIGS. 3 and 4 was subjected to the same test under the same conditions as set forth above, the consumption of the anode was uniform as indicated in FIG. 1(b). Moreover, the anode thickness was only reduced to approximately mm.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention as set forth in the appended claims.

We claim:

1. In mercury-process electrolysis of brine supplied into an electrolytic cell into which mercury is introduced through a mercury inlet below and across substantially the full width of a transverse wall of the electrolytic cell, the method of operating said cell which comprises supplying said brine as a falling laminar-flow curtain descending along substantially the entire inner surface of said trarisverse wall and reaching the mercury thus introduced thereby to wash the entire upper surface of said mercury and the region thereabove.

2. In a mercury-process brine electrolytic cell having a transverse inlet wall and adapted to contain an amount of electrolyte which fills the cell to a normal surface level, the improvement which comprises a supply brine flow inlet at the top and extending horizontally across substantially the full width of said transverse wall, said supply brine flow inlet being discontinuous, and comprising a horizontal in-line row of a plurality of divided parts.

3. In a mercury-process brine electrolytic cell having a transverse inlet wall and adapted to contain an amount of electrolyte which fills the interior of the cell to a normal electrolyte surface level, brine supply means comprising, in combination, a brine flow inlet at the top of and extending horizontally across substantially the full width of said transverse wall at a position substantially higher than said normal electrolyte surface level and a brine supply passageway disposed at the top of the transverse wall and on the side of the transverse wall opposite the interior of the cell and opening into said brine flow inlet, said supply brine flow inlet being discontinuous, and comprising a horizontal in-line row of a plurality of divided parts.

4. In a mercury process brine electrolytic cell adapted to contain an amount of electrolyte which fills the interior of the cell to a normal electrolyte surface level, the combination of a substantially vertical transverse inlet wall extending across the width of the cell at the inlet end of said cell, said inlet wall having a mercury inlet opening into the cell at the bottom of the wall and extending horizontally across substantially the full width of the cell, said inlet wall further having a brine flow inlet in the upper portion of the wall substantially above said normal electrolyte surface level and a brine supply passageway disposed at the upper portion of said inlet wall and opening into said brine flow inlet, said brine flow inlet extending horizontally across substantially the full width of said cell and being sufficiently far above said normal electrolyte surface level that when the cell is filled to said normal surface level with brine and a falling laminar-flow curtain of brine is flowed through said inlet and down along substantially the entire inner surface of said transverse wall, it will flow down through the brine to the mercury inlet and wash the upper surface of the mercury fiowing into the cell through said mercury inlet.

References Cited UNITED STATES PATENTS 619,349 2/1899 Riecken 204-220 1,238,600 8/1917 Trumbull 20499 2,226,784 12/1940 Sorensen 20499 2,232,128 2/1941 Muller 204-99 3,216,916 11/1965 Locke 204-196 TA-HSUNG TUNG, Primary Examiner US. Cl. X.R. 2042l9, 250 

